Mixed ligand nickel(II) complexes as catalysts for alkane hydroxylation ...

Inorganica Chimica Acta 407 (2013) 98–107

Contents lists available at ScienceDirect

Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica

Mixed ligand nickel(II) complexes as catalysts for alkane hydroxylation using m-chloroperbenzoic acid as oxidant Muniyandi Sankaralingam a, Prabha Vadivelu b, Eringathodi Suresh c, Mallayan Palaniandavar a,d,⇑ a

School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu, India Chemical Sciences and Technology Division, National Institute for Interdisciplinary Science and Technology, CSIR, Trivandrum 695019, India c Analytical Science Discipline, Central Salt and Marine Chemicals Research Institute, Bhavnagar 364 002, India d Department of Chemistry, Central University of Tamil Nadu, Thiruvarur 610004, Tamil Nadu, India b

a r t i c l e

i n f o

Article history: Received 10 February 2013 Received in revised form 11 July 2013 Accepted 18 July 2013 Available online 27 July 2013 Keywords: Nickel complexes Catalysis Alkane hydroxylation Nickel-oxo intermediate

a b s t r a c t A new family of nickel(II) complexes of the type [Ni(PA)(L)(CH3CN)n]BPh4 1–5, where n = 1, 2, H(PA) is 2-picolinic acid and L is N,N0 -tetramethylethylenediamine (L1) 1, N,N0 ,N00 -pentamethyldiethylenetriamine (L2) 2, 2,20 -bipyridine (L3) 3, 1,10-phenanthroline (L4) 4 or 2,9-dimethyl-1,10-phenanthroline (L5) 5, has been isolated and characterized using CHN analysis, UV–Vis spectroscopy and ESI-MS. The complex [Ni(PA)(L2)(CH3CN)](BPh4) 2 possesses a distorted octahedral coordination geometry in which Ni(II) is chelated to 2-picolinate anion and L2. DFT calculations show that trans isomers of 3–5 are more stable than cis isomers by ca. 4.0 kJ/mol. In contrast, cis-1 is more stable than trans-1 by 15.8 kJ/mol. The complexes catalyze the hydroxylation of cyclohexane efficiently in presence of m-CPBA as oxidant with 244–569 turnover numbers and good alcohol selectivity (A/K, 3.4–7.0). Adamantane is oxidized to 1-adamantanol, 2-adamantanol and 2-adamantanone with varying bond selectivity (3°/2°, 9.3–14.2) while cumene is selectively oxidized to 2-phenyl-2-propanol. Upon replacing bidentate L1 by tridentate L2 or strongly p-back bonding phen the catalytic activity increases. In contrast, phen is replaced by non-planar bpy or 2,9-dmp with sterically hindering methyl groups the catalytic activity decreases. Thus ligand denticity, Lewis acidity of Ni(II) center and p-back bonding determine the catalytic activity. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction In recent years catalytic oxidation of saturated hydrocarbons under mild conditions is attracting great attention among academics and industries as selective hydroxylation of alkanes is an important and difficult chemical transformation. Significant efforts have been made to design efficient catalysts for the hydroxylation of alkanes as conventional processes usually require high temperatures and high pressures [1–6]. In nature, there are many widely investigated iron-proteins such as methane monooxygenases, which carry out the unique and selective oxidation of methane to methanol using dioxygen [7–16]. Inspired by the interesting functional aspects of such metalloenzymes, bioinorganic chemists have paid much attention to isolation of biomimetic model complexes containing metal ions, and study of reproducing the interesting chemical transformations catalyzed by the enzymes and have made significant advancements in their efforts. Also, another target is the understanding of enzyme mechanisms, which may lead to new insights into biocatalysis and development of new industrial ⇑ Corresponding author. Tel.: +91 9489079402; fax: +91 4366225312. E-mail addresses: (M. Palaniandavar).

[email protected],

[email protected]

0020-1693/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2013.07.031

catalysts. So far, several bio-inspired iron(II)/(III) complexes have been investigated as catalysts for hydroxylation, epoxidation and sulfoxidation reactions [17–30]. Though iron complexes are considered to be one of the most promising catalysts for the industrially important alkane oxidation reactions, a variety of metal-based homogeneous catalysts for the oxidation reactions has been reported by replacing iron with different transition metals [31–53]. Very recently, nickel has emerged as an attractive transition metal for the development of catalysts for the industrially important hydroxylation of alkanes. Earlier several oxo-bridged dinuclear Ni(II) complexes have been reported to be involved in oxygen activation chemistry [54–68]. Very recently, Itoh et al. have studied the effect of ligand and counter anion of many Ni(II) complexes and found that [Ni(TPA)(OAc)(H2O)](BPh4), where TPA is tris(pyrid-2ylmethyl)amine, is a very efficient and robust turnover catalyst for the catalytic oxidation of cyclohexane with m-chloroperbenzoic acid (m-CPBA) as oxidant, exhibiting a high alcohol selectivity in the reaction [69–71]. They suggested the involvement of the highly reactive nickel-oxo (Ni = O+) intermediate species rather than an auto-oxidation type free radical species in the catalytic cycle [70]. Hikichi et al. have crystallized the nickel(II) alkylperoxo complex [NiII(Tpipr)(OOtBu)], where Tpipr is hydrotris(3,5-di-2-propylpyrazolyl)borate, and studied its catalytic oxidation activity

M. Sankaralingam et al. / Inorganica Chimica Acta 407 (2013) 98–107

towards the oxidation of alkanes [72]. Very recently, we have investigated a series of nickel(II) complexes of tripodal 4N [73] and 5N [74] ligands as catalysts for alkane hydroxylation reaction using m-CPBA as the oxidant and found that the total turnover number (TON) varies linearly with the metal–ligand covalency parameter (b), supporting the preference of the nickel-oxo (Ni = O+) intermediate species and the role of metal–ligand covalent bonding [73]. In this study, we explore the use of new mononuclear mixed ligand Ni(II) complexes supported by picolinic acid [H(PA)] as the primary ligand and bidentate N,N0 -tetramethylethylenediamine (L1), 2,20 -bipyridine (L3), 1,10-phenanthroline (L4) and 2,9-dimethyl-1,10-phenanthroline (L5) and the tridentate N,N0 ,N00 -pentamethyldiethylenetriamine (L2) as co-ligands (Scheme 1) for alkane hydroxylation using m-CPBA as oxidant. So far Ni(II) complexes of phenolate and nitrogen donor ligands have been used as catalysts for alkane hydroxylation reactions. Now we aim at constructing more efficient and improved alcohol selective nickel(II) catalysts for alkane hydroxylation using the strongly coordinating picolinic acid as the primary ligand and different 2N and 3N nitrogen donor co-ligands using m-CPBA as the oxidant. Also, we wish to investigate the effect of supporting ligands in determining the formation and stabilization of the still unclear reactive intermediate species involved in the alkane hydroxylation reaction. The incorporation of p-back bonding L3, L4 and L5 ligands are expected to stabilize the reactive nickel-oxo intermediate species to different extents. All the present complexes catalyze the hydroxylation of alkanes like cyclohexane, adamantane and cumene efficiently (244–569 TON) with good alcohol selectivity for cyclohexane (A/K, 5.1–7.0) within 4 h. The catalytic activity increases when the bidentate ligand L1 in 1 is replaced by the tridentate L2 ligand and by phen ligand to obtain 2 and 4 respectively. However, when phen is replaced with bpy (3) and 2,9-dmp (5), the catalytic activity decreases because of destabilization of the intermediate [(PA)(L) (CH3CN)Ni-O] radical species. The phen complex 4 selectively catalyzes the oxidation of adamantane to 1-adamantanol while the remaining complexes oxidize adamantane to 1-adamantanol, 2-adamantanol and 2-adamantanone (3°/2°, 9.3–14.2), and cumene is selectively oxidized to 2-phenyl-2-propanol. 2. Experimental

99

Mumbai), acetonitrile and tetrahydrofuran (Merck, India) were distilled before use. 2.2. Synthesis of Ni(II) complexes 2.2.1. [Ni(PA)(L1)(CH3CN)2](BPh4) 1 A methanol solution (5 mL) of Ni(ClO4)26H2O (0.365 g, 1 mmol) was added to a mixture of picolinic acid (0.123 g, 1 mmol), L1 (0.116 g, 1 mmol) and triethylamine (0.10 g, 1 mmol) in methanol solution (5 mL) with stirring at room temperature. After stirring the resulting indigo colored solution for 30 min, NaBPh4 (0.342 g, 1 mmol) dissolved in acetonitrile (5 mL) was added to this mixture. The mixture was stirred for additional 30 min. The blue precipitate obtained was filtered off and washed with cold methanol and then diethylether. Yield, 0.57 g, 82%. ESI-MS: m/z 296.20 [(M–2CH3CN– BPh4)+] (Fig. S1). Anal. Calc. C40H46N5O2BNi: C, 68.80; H, 6.64; N, 10.03. Found: C, 68.84; H, 6.61; N, 9.98%. 2.2.2. [Ni(PA)(L2)(CH3CN)](BPh4) 2 The procedure employed for the preparation of 1 was used for 2 also and N,N0 ,N00 -pentamethyldiethylenetriamine (L2, 0.173 g, 1 mmol) L2 was used instead of L1. The blue precipitate obtained was filtered off and washed with cold methanol and diethylether. Single crystals suitable for X-ray crystallographic analysis were obtained by slow evaporation of CH3CN/DCM solution of the complex. Yield, 0.62 g, 87%. ESI-MS: m/z 353.20 [(M–CH3CN–BPh4)+] (Fig. S2). Anal. Calc. C41H50N5O2BNi: C, 68.93; H, 7.05; N, 9.80. Found: C, 68.89; H, 7.11; N, 9.72%. 2.2.3. [Ni(PA)(L3)(CH3CN)2](BPh4) 3 The procedure employed for the preparation of 1 was used for 3 also and 2,20 -bipyridine (L3, 0.156 g, 1 mmol) was used instead of L1. Yield, 0.59 g, 80%. ESI-MS: m/z 336.07 [(M–2CH3CN–BPh4)+] (Fig. S3). Anal. Calc. for C44H38N5O2BNi: C, 71.58; H, 5.19; N, 9.49. Found: C, 71.52; H, 5.13; N, 9.44%. 2.2.4. [Ni(PA)(L4)(CH3CN)2](BPh4) 4 The procedure employed for the preparation of 1 was used for 4 also and 1,10-phenanthroline (L4, 0.198 g, 1 mmol) was used instead of L1. Yield 0.58 g, 76%. ESI-MS: m/z 360.07 [(M–2CH3CN– BPh4)+] (Fig. S4). Anal. Calcd. for C46H39N5O2BNi: C, 72.38; H, 5.15; N, 9.17. Found: C, 72.32; H, 5.12; N, 9.21%.

2.1. Materials 0

Nickel(II) perchlorate hexahydrate, picolinic acid, N,N -tetramethylethylenediamine, N,N0 ,N00 -pentamethyldiethylenetriamine, 2,9-dimethyl-1,10-phenanthroline, adamantane, cumene, sodium tetraphenylborate, m-choloroperbenzoic acid (Aldrich), 2,20 -bipyridine, 1,10-phenanthroline (Alfa Aesar), triethylamine, dichloromethane, diethylether (Merck, India) and cyclohexane (Ranbaxy) were used as received. Methanol (Sisco Research Laboratory,

O

2.3. Reactivity studies

OH

N

N

N L3

N L1

PA

N

2.2.5. [Ni(PA)(L5)(CH3CN)2](BPh4) 5 The procedure employed for the preparation of 1 was used for 5 also and 2,9-dimethyl-1,10-phenanthroline (L5, 0.208 g, 1 mmol) was used instead of L1. Yield 0.56 g, 70%. ESI-MS: m/z 388.07 [(M–2CH3CN–BPh4)+] (Fig. S5). Anal. Calcd. for C48H43N5O2BNi: C, 72.85; H, 5.48; N, 8.85. Found: C, 72.81; H, 5.42; N, 8.90%. Caution! Perchlorate salts of the compounds are potentially explosive. Only small quantities of these compounds should be prepared and suitable precautions should be taken when they are handled.

N

N L4

N

N L2

N

N

N L5

Scheme 1. Structures of ligands employed in the study.

The oxidation of alkanes was carried out at room temperature under research grade nitrogen atmosphere. In a typical reaction, Ni(II) complex (0.35 10–3 mmol dm3) was added to a mixture of alkanes (2.45 mol dm3) and oxidant m-CPBA (0.35 mol dm3) in CH2Cl2:CH3CN mixture (3:1 v/v). After 4 h the reaction mixture was quenched with triphenylphosphine, the reaction mixture was filtered over a silica column and then eluted with diethylether. An internal standard (bromobenzene) was added at this point and the solution was subjected to GC analysis. The mixture of organic products were identified by Agilent GC–MS and quantitatively analyzed by HP 6890 series GC equipped with HP-5 capillary column

100


(30 m 0.32 mm 2.5 lm) using a calibration curve obtained with authentic compounds. All of the products were quantified using GC (FID) with the following temperature program: injector temperature 130 °C; initial temperature 60 °C, heating rate 10 °C min1 to 130 °C, increasing the temperature to 160 °C at a rate of 2 °C min1, and then increasing the temperature to 260 °C at a rate of 5 °C min1; FID temperature 280 °C. GC–MS analysis was performed under conditions identical to those used for GC analysis. The averages of three measurements are reported.

Table 1 Crystallographic data for complex 2. 2 Empirical formula Formula weight (g mol1) Crystal habit, color Crystal system Crystal size Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V(Å3) Z qcalc (g cm3) F(0 0 0) T (K) No. of Reflections collected No. of unique reflections Radiation (Mo Ka) (Å) Goodness-of-fit on (GOF) F2 Number of refined parameters R1/wR2 [I > 2r(I)]a R1/wR2 (all data)

2.4. Physical measurements Elemental analyses were performed on a Perkin Elmer Series II CHNS/O analyzer 2400. The electronic spectra were recorded on an Agilent 8453 diode array spectrophotometer. ESI-MS analysis was recorded on a Micro mass Quattro II triple quadrupole mass spectrometer. GC–MS analysis was performed on an Agilent GC–MS equipped with 7890A GC series (HP-5 capillary column) and 5975C inert MSD. The products were quantified by using Hewlett Packard (HP) 6890 Gas Chromatograph (GC) series equipped with a FID detector and a HP-5 capillary column (30 m 0.32 mm 2.5 lm). All the catalytic reactions were performed under research grade nitrogen atmosphere using standard Schlenk line technique.

a

C41H50BN5O2Ni 714.38 pink orthorhombic 0.43 0.38 0.33 mm Pna21 23.107(2) 13.5882(12) 11.9638(10) 90 90 90 3756.4(6) 4 1.263 1520 110(2) 21 232 8620 0.71073 1.037 457 0.0381/0.0989 0.0434/0.1022

R1 = [R(||Fo| |Fc||)/R|Fo|]; wR2 = {[R(w(Fo2 Fc2)2)/R(wFo4)]1/2}.

2.5. Crystal data collection and structure refinement The diffraction experiments were carried out on a Bruker SMART APEX diffractometer equipped with a CCD area detector. High quality crystals, suitable for X-ray diffraction was chosen after careful examination under an optical microscope. Intensity data for the crystal was collected using Mo Ka (k = 0.71073 Å) radiation on a Bruker SMART APEX diffractometer equipped with CCD area detector at 110 K. The data integration and reduction was processed with SAINT [75] software. An empirical absorption correction was applied to the collected reflections with SADABS [76]. The structure was solved by direct methods using SHELXTL [77] and refined on F2 by the full-matrix least-squares technique using the SHELXL-97 [78] package. Other non-hydrogen atoms were located in successive difference Fourier syntheses. The final refinement was performed by full-matrix least-squares analysis. Hydrogen atoms attached to the ligand moiety were located from the difference Fourier map. Crystal data and additional details of the data collection and refinement of the structure are presented in Table 1. The selected bond lengths and bond angles are listed in Table 2.

3. Results and discussion

Table 2 Selected bond lengths [Å] and bond angles [°] for 2. 2 Bond lengths (Å) Ni(1)–N(1) Ni(1)–N(2) Ni(1)–N(3) Ni(1)–N(4) Ni(1)–N(5) Ni(1)–O(1) Bond angles (°) N(1)–Ni(1)–N(2) N(1)–Ni(1)–N(3) N(1)–Ni(1)–N(4) N(1)–Ni(1)–N(5) N(2)–Ni(1)–N(3) N(2)–Ni(1)–N(4) N(2)–Ni(1)–N(5) N(3)–Ni(1)–N(4) N(3)–Ni(1)–N(5) N(4)–Ni(1)–N(5) O(1)–Ni(1)–N(1) O(1)–Ni(1)–N(2) O(1)–Ni(1)–N(3) O(1)–Ni(1)–N(4) O(1)–Ni(1)–N(5)

2.229(2) 2.105(2) 2.210(2) 2.0706(18) 2.0569(19) 2.0274(16) 83.04(8) 165.37(8) 97.09(7) 91.02(8) 82.59(9) 173.68(8) 93.72(8) 96.90(8) 92.62(8) 92.60(7) 88.35(7) 93.53(8) 89.83(8) 80.17(7) 172.60(7)

3.1. Syntheses and characterization of nickel(II) complexes 3.2. Structures of nickel(II) complexes The mixed ligand nickel(II) complexes [Ni(PA)(L)(CH3CN)x] (BPh4) 1–5 were isolated by treating Ni(ClO4)26H2O with a mixture of one equivalent each of H(PA), the corresponding co-ligands L1–L5 and triethylamine in methanol, adding acetonitrile with stirring and then treating the reaction mixture with stoichiometric amount of NaBPh4 dissolved in acetonitrile. All the complexes were characterized by using elemental analysis, electronic spectroscopy and mass spectrometry. The formulation of the complexes based on elemental analysis is confirmed by determining the X-ray crystal structure of 2 (Scheme 2, cf. below). Hence, the variations of ligand donor functionalities of the complexes are expected to play an important role in determining the reactivity and stability of the reactive intermediate species and eventually the catalytic activity of the complexes.

3.2.1. Description of X-ray structure of [Ni(PA)(L2)(CH3CN)](BPh4) 2 The molecular structure of [Ni(PA)(L2)(CH3CN)](BPh4) 2 is shown in Fig. 1 together with the atom numbering scheme and the selected bond lengths and bond angles are collected in Table 2. The complex molecule contains a NiN5O coordination sphere with a distorted octahedral coordination geometry constituted by the three tertiary amine nitrogen atoms of L2 ligand, and the nitrogen and oxygen atoms of picolinic acid and the remaining coordination site trans to the carboxylate oxygen atom is occupied by an acetonitrile molecule. The meridional coordination of the tridentate ligand L2 is similar that in mixed ligand copper(II) and certain iron(II) complexes reported already [79,80]. The Ni–Npy (2.0451– 2.1692 Å) and Ni–Namine (1.926–2.196 Å) bond distances are in

101


N

N

S N

O Ni S

N

N

N S

a 2

N

O

Ni

Ni

N

N

O

N

O

N

N

Ni S

N

S

S

b trans

c cis

d

1, 3, 4, 5

1, 3, 4, 5

5

Scheme 2. Possible coordination geometries a–d (S = CH3CN) for 1–5.

N1 O1

N4

N2 Ni

N5

N3

Fig. 1. ORTEP diagram of [Ni(PA)(L2)(CH3CN)](BPh4) 2 showing 50% probability thermal ellipsoids and the labeling scheme for selected atoms. All hydrogen atoms are omitted for clarity.

the ranges observed for previously reported nickel(II) complexes (Ni–Npy, 2.0706; Ni–Namine, 2.105–2.229 Å) [69–71,73,74]. The Ni–Npy bond (2.0706(2) Å) is shorter than all the Ni–Namine bonds (2.229(2), 2.210(2), 2.105(2) Å) due to sp2 and sp3 hybridizations of the pyridyl and tertiary amine nitrogen atoms respectively. Also, the terminal Ni–Namine bonds (2.229(2), 2.210(2) Å) are longer than the central Ni–N2amine bond (2.105(2) Å), as expected. The Ni– N5ACN bond (2.0569(2) Å) is shorter than all other Ni–N bonds due to sp hybridization of acetonitrile nitrogen (cf. above). The Ni–O1 bond (2.0274(16) Å) is shorter than all the Ni–N bonds indicating that the carboxylate oxygen is coordinated very strongly to the nickel(II) centre. For 2 the bond angles 80.17–97.09° and 165.37–173.68° deviate from the ideal octahedral angles of 90° and 180° respectively revealing the presence of significant distortion in the Ni(II) coordination geometry.

3.2.2. Structures of nickel(II) complexes: density functional theory (DFT) calculations A DFT study (using g09 program) [81] has been performed to investigate the geometrical parameters of 1–5 (computed geometries are shown in Scheme 3 and Fig. S6). Depending upon the solvent/acetonitrile coordination, the complexes may exist in both cis and/or trans forms (Scheme 2). Initially, as a bench mark calculation,1 the crystal structure of 2 was optimized. The computed geometry (20 ) is in good agreement with the experimentally determined crystal structure, except a slight elongation in bond lengths ranging from 0.03 to 0.1 Å (Fig. S6). Hence, the same computational methodology has been followed for optimizing the geometries of other 1 All calculations were run with g09 program employing B3LYP basis function. The metal center (Ni) was described by the LANL2DZ basis set along with the associated RECPs. The rest of the atoms were described by the 6–31G⁄⁄ basis set.

complexes (Scheme 2). For the complexes 1, 3 and 4 in both cis and trans isomeric forms, (see the supporting information for the optimized structures Fig. S6), the equatorial donors are nearly co-planar and are shown by the torsion angle of Ni–Ni–O1–N3 and Ni–Ni–N4– N3 which is equal to ca. 180°. The remaining axially disposed donors are slightly distorted from the axial coordination plane (N1–Ni–N2– N3 and Ni–Ni–N5–N3 lies around 170–173° respectively). This indicates that the complexes 1, 3 and 4 exhibit a slightly distorted octahedral geometry. In contrast, the complex 5 possesses an octahedral geometry more distorted than those of 1, 3 and 4. In the case of trans-5, the equatorial planes are more distorted (N1–Ni–O1–N3 and N1–Ni–N4–N3 of 168°) whereas the axial planes are nearly co-planar. Also, in comparison with the analogous trans-4 structure (the benzene ring in the equatorial plane), the benzene rings in trans-5 are twisted and are distorted from the equatorial plane by ca. 165° (Ni–N3–C1–C2, 162°, Ni–N4–C3–C4, 168°) leading to a twisted boat-like geometry (Scheme 3). This is due to the steric hindrance of the incorporated methyl group on phen ring at 2,9 positions towards Ni(II) coordination in 5. Similar to 1, 3 and 4, cis-5 also shows a co-planar equatorial and distorted axial planes (here the distortion is higher, around 169°). However, as seen with trans-5, here also the benzene rings in phen are twisted from the equatorial plane (by Ni–N3–C1–C2 of 163.5° and Ni–N4–C3–C4 of 159.4°). Overall, a comparison of computational structures of 1–5 indicate that both the cis- and trans-5 are more distorted than those of 1–4 and even trans-5 looks like a twisted boat in the equatorial plane. So, the complex 5 has the tendency to release one of the coordinated solvent (CH3CN) molecules to obtain a stable fivecoordinate square pyramidal geometry (d, Scheme 2). A comparison of the energetics reveals that the trans isomer of 3–5 is more stable than the cis isomer by ca. 4.0 kJ/mol; however, cis-1 isomer is more stable than trans-1 isomer by 15.8 kJ/mol. 3.3. Electronic spectral properties The electronic spectral data of all the Ni(II) complexes are summarized in Table 3 and the typical electronic absorption spectrum of 2 is shown in Fig. 2. In DCM:ACN (3:1 v/v) solvent mixture, all the nickel(II) complexes exhibit two broad absorption bands in the ranges 545–585 and 890–960 nm together with a very weak shoulder in the range 772–794 nm. The observed band positions are well matches with the previously reported octahedral Ni(II) complexes [70,73]. The lower energy band is assigned to 3A2g ? 3T2g(F) (m1) transition, and the higher energy band to the 3A2g ? 3T1g(F) (m2) transition in Ni(II) located in an octahedral environment. The intense shoulder observed around 360 nm in the UV region is assigned to 3A2g ? 3T1g(P) (m3) transition while the weak shoulder in the range 772–794 nm to 3A2g ? 1E1g(D) spin-forbidden transition [82,83]. By fitting the observed m1 and m2 band positions into the quadratic equation connecting the energies of both m1 and m3 transitions the metal–ligand covalency parameter (b) was calculated

102


N1-Ni-O1-N3 = 167.6 N1-Ni-N4-N3 = 167.5 N1-Ni-N2-N3 = 177.0 N1-Ni-N5-N3 = 177.0 Ni-N3-C1-C2 = 162.0 Ni-N4-C3-C4 = 167.6

C1

N2 O1 2.17 1.98 2.17

Ni

2.17

C2 N2

1.98 O1

C2

N1 2.11

N4 C3

N5

C1 C3

2.18

2.18 N3

2.20

N1

N1-Ni-O1-N3 = 179.0 N1-Ni-N4-N3 = 179.0 N1-Ni-N2-N3 = 169.0 N1-Ni-N5-N3 = 169.0 Ni-N2-C1-C2 = 163.5 Ni-N3-C3-C4 = 159.4

2.16 N3 Ni

C4 2.19 N4

2.21

C4

N5

trans-5 E =-1521.28529923 a.u

cis-5 E = -1521.28383172 a.u

Scheme 3. Computed geometries of 5 in both cis and trans forms. Bond lengths in Å and torsion angles in deg.

Table 3 UV–Vis spectral data (kmax in nm; e in M1 cm1 in parenthesis) of Ni(II) complexes 1–5 in DCM:ACN solvent mixture (3:1 v/v) at 25.0 °C. 3

A2g ? 3T1g(P) (m3)

Complex

1 2 3 4 5

3

Found

Calcda

354 (36) 360 (60) – – –

334 335 322 328 359

A2g ? 3T1g(F) (m2)

576 (9) 582 (11) 545 (8) 555 (13) 585 (13)

3

A2g ? 1E1g (D)

778 794 772 772 788

(5) (7) (8) (5) (5)

3

A2g ? 3T2g(F) (m1)

B0

b

947 (11) 960 (16) 890 (11) 904 (8) 936 (7)

1041 1055 1045 1018 856

3.5 2.3 3.2 5.7 20.7

b (%)

a,b

Calculated by solving the quadratic equation and using B as 1080 cm–1.

[83]. The calculated band positions of m3 band agree with the observed ones (Table 3), which confirms the band assignments and hence the octahedral coordination geometry for Ni(II) complexes in solution also. The values of metal–ligand covalency parameter (b) for the present complexes fall in the range b, 2.3-5.7. However, the 2,9-dmp complex 5 shows a very high b value of 20.7; this is consistent with the above computational study, which reveals that the octahedral geometry for 5 is unstable and tends to adopt a stable five-coordinate square pyramidal geometry in solution [70,84].

Absorbance

0.3 ν1

0.2 ν2

0.1

0 350

500

650 800 Wavelength (nm)

950

1100

Fig. 2. Electronic absorption spectra of [Ni(PA)(L2)(CH3CN)](BPh4) (1.0 102 M) in DCM:ACN solution.

3.4. Catalytic oxidation of alkanes The experimental conditions and results of catalytic oxidation of different alkanes for all the nickel(II) complexes using different oxidants are summarized in Tables 4–7. The conversion of alkanes

Table 4 Conversion of cyclohexane catalyzeda by 2 with time. S.No.

Time (min.)

Cyclohexane (TON) -ol

1 2 3 4 5 6 7 a

30 60 120 180 240 720 1440

b

159 219 342 399 498 510 572

b

-one

e-caprolactone

15 13 8 10 17 18 34

20 32 35 40 54 56 67

Total TONc

Chlorobenzene

A/Kd

Yielde (%)

194 264 385 549 569 584 673

149 192 326 383 491 502 552

4.5 4.9 7.9 8.0 7.0 6.9 5.7

19.4 26.4 38.5 54.9 56.9 58.4 67.3

Reaction conditions: Catalyst (0.35 103 mmol dm3), Substrate (2.45 mol dm3), Oxidant (0.35 mol dm3) in DCM:ACN solvent mixture (3:1 v/v). -ol = cyclohexanol and -one = cyclohexanone. c Total TON = No. of mmol of product/No. of mmol of catalyst. d A/K = TON of -ol/ (TON of -one + TON of e-caprolactone). e Yield based on the oxidant. b

103

M. Sankaralingam et al. / Inorganica Chimica Acta 407 (2013) 98–107 Table 5 Products of oxidation of cyclohexane catalyzeda by Ni(II) complexes. Complex

Cyclohexane (TON) b

Blankf 1 2 3 4 5

-ol

-one

3 487 498 208 489 250

1 16 17 12 20 13

b

Total TONc

Chlorobenzene

A/Kd

Yielde (%)

4 557 569 244 585 262

461 491 193 446 202

3.0 7.0 7.0 5.8 5.1 6.8

0.4 55.7 56.9 24.4 58.5 26.2

e-caprolactone – 54 54 24 76 24

a

Reaction conditions: Catalyst (0.35 103 mmol dm3), Substrate (2.45 mol dm–3), Oxidant (0.35 mol dm–3) in DCM:ACN solvent mixture (3:1 v/v). -ol = cyclohexanol and -one = cyclohexanone. c Total TON = No. of mmol of product/No. of mmol of catalyst. d A/K = TON of -ol/ (TON of -one + TON of e-caprolactone). e Yield based on the oxidant. f Blank = Substrate (2.45 mol dm3), Oxidant (0.35 mol dm3) in DCM:ACN solvent mixture (3:1 v/v). b

into hydroxylated products was quantified based on gas chromatographic analysis by using authentic samples and an internal standard (bromobenzene). The catalytic ability of the complexes towards oxidation of alkanes like cyclohexane, adamantane and cumene was studied by using m-CPBA, H2O2 and t-BuOOH as oxidants in dichloromethane/acetonitrile solvent mixture (3:1 v/v) at room temperature under nitrogen atmosphere. When H2O2/tBuOOH was used as the oxidant only trace amounts of the oxidized products are formed revealing that the complexes are not effective at all as catalysts in the presence of these oxidants. In the presence of m-CPBA the oxidation of cyclohexane proceeds to give cyclohexanol as the major product (A, 20.8–49.8%) and cyclohexanone (K, 1.2–1.7%) and e-caprolactone (2.4–5.4%) as the minor products. It has been previously reported that in the absence of any metal catalyst m-CPBA is a very strong oxidizing agent for oxidation of cyclohexane and adamantane to the corresponding alcohols and ketones; however, vigorous reaction conditions like very high concentration of m-CPBA, high temperature and long reaction time are employed [85]. In control reactions performed in the absence of the complexes with m-CPBA as oxidant only very small amounts of the oxidized products are observed for all the substrates (Cyclohexane, 4 TON; Adamantane, 9 TON; Cumene, 5 TON). This reveals that all the complexes act as catalysts towards oxidation of alkanes into alcohols. It is noted that m-CPBA is a quantitative reagent for converting cyclic ketones into their corresponding lactones (Baeyer–Villiger oxidation) in the absence of a metal catalyst, e-caprolactone is not the metal-catalyzed product [86]. The latter is the over oxidized product of oxidation of cyclohexanone by the excess or unreacted m-CPBA. Also, interestingly, only 50% of the oxidized products are formed under atmospheric conditions, revealing the absence of a free radical auto-oxidation reaction.

The complex 1 catalyses the oxidation of cyclohexane to 557 TON (487 TON of cyclohexanol, 16 TON of cyclohexanone, 54 TON of e-caprolactone) with an alcohol to ketone selectivity (A/ K) of 7.0. As proposed earlier, we suggest that m-CPBA binds with Ni(II) center in the catalyst by replacing a labile acetonitrile molecule to form the adduct species [NiII(L)(PA)(OOCOC6H4Cl)] (Scheme 4), which undergoes either (a) homolysis leading to the formation of [(L)(PA)NiII-O]/[(L)(PA)NiIII = O] intermediate species and m-chlorobenzoic acid radical or (b) heterolysis leading to the formation of [(L)(PA)NiIV = O] intermediate species and benzoic acid [70]. The species [(L)(PA)NiII-O]/[(L)(PA)NiIII = O] is then involved in selective hydroxylation of alkanes while m-chlorobenzoic acid radical undergoes decarboxylation to form chlorobenzene in more than 79–86% yield. The observation of chlorobenzene

Table 7 Oxidation product of cumene catalyseda by Ni(II) complexes. Complex

2-Phenyl-2-propanol

Total TONb

Yieldc (%)

Blankd 1 2 3 4 5

5 257 456 136 428 182

5 257 456 136 428 182

0.5 25.7 45.6 13.6 42.8 18.2

a Reaction conditions: catalyst (0.35 103 mmol dm3), Substrate (2.45 mol dm3), Oxidant (0.35 mol dm3) in DCM:ACN solvent mixture (3:1 v/v). b Total TON = No. of mmol of product/No. of mmol of catalyst. c Yield based on the oxidant. d Blank = Substrate (2.45 mol dm3), Oxidant (0.35 mol dm3) in DCM:ACN solvent mixture (3:1 v/v).

Table 6 Products of oxidation of adamantane catalyzeda by Ni(II) complexes. Complex

1-adol Blankf 1 2 3 4 5 a

Total TONc

Adamantane (TON)

6.2 425 482 381 471 280

b

2-adol 2.4 123 113 98 141 37

b

2-adone 0.4 – 42 23 – 22

b

Selectivityd 3°/2°

9 548 637 502 612 339

6.6 10.4 9.3 9.4 10.0 14.2

Reaction conditions: Catalyst (0.2 103 mmol dm3), Substrate (0.4 mol dm3), Oxidant (0.2 mol dm3) in DCM:ACN solvent mixture (3:1 v/v). 1-adol = 1-adamantanol, 2-adol = 2-adamantanol and 2-adone = 2-adamantanone. c TON = No. of mmol of product/No. of mmol of catalyst. d 3°/2° = (TON of 1-adol 3)/(TON of 2-adol + TON of 2-adone). e Yield based on the oxidant. f Blank = Substrate (0.4 mol dm3), Oxidant (0.2 mol dm3) in DCM:ACN solvent mixture (3:1 v/v). b

Yielde (%)

0.9 54.8 63.7 50.2 61.2 33.9

104


O Cl

HO O II

(PA)(L)Ni

[(PA)(L)Ni(CH3CN)x] x = 1 for 1; x = 2 for 2 - 5

II

O

O Cl

O O

ho m

lys is

[(PA)(L)Ni

II

O

ro

oly s

is

Cl

O

te

-CO2

he

Cl

O]

IV

(PA)(L)Ni

or III O] [(PA)(L)Ni

Cl

HO

O

m-CPBA

O

O O

OH

OH

O

O m-CPBA

m-CPBA

II

[(PA)(L)Ni(CH3CN)x]

O

II

[(PA)(L)Ni(CH3CN)x]

Scheme 4. Proposed mechanism of alkane hydroxylation for 1–5.

Cyclohexanol

Cyclohexanone

Caprolactone

700 600

Total TON

500 400 300 200 100 0 30

60

120

180 240 Time (min.)

720

1440

Fig. 3. Bar chart representation of conversion of cyclohexane catalyzed by 2 with time in DCM:ACN solvent mixture (3:1 v/v) at room temperature.

supports the involvement of the intermediate species [(L)(PA)NiIIO]/[(L)(PA)NiIII = O] in catalysis and indicates that m-chlorobenzoate radical is not the reactive intermediate because it readily undergoes decarboxylation rather than hydrogen abstraction from alkane. The formation of the remaining alkane hydroxylated products (14–19%) reveals that the gate for formation of another intermediate species, possibly [(L)(PA)NiIV = O], is unlocked. Adducts with m-CPBA, similar to [NiII(PA)(L)(OOCOC6H4Cl)], and reactive intermediates, have been proposed previously by Itoh et al. to illustrate the catalytic activity of complexes of the type [Ni(L)(X)(X0 )], where L is a tetradentate ligand and X and X0 are OAc, NO 3,

m-CBA or phenolate [69]. By invoking the formation of such an intermediate, we have successfully illustrated the trends in catalytic activity of nickel(II) complexes of a new family of 4N and 5N ligands towards alkane hydroxylation [73,74]. The proposed involvement of the intermediate adduct species [NiII(L) (PA)OOCOC6H4Cl)] is supported by the successful isolation and X-ray crystal structure determination of the nickel(II) alkylperoxo complex [NiII(Tpipr)(OOtBu)], which has been actually employed for oxidation of alkanes [72]. Also, Kallol Ray et al. have used EPR and ESI-MS techniques to characterize the intermediate species [(TMG3tren)Ni-O(H)]n+, where n = 2 for oxo, and n = 3 for hydroxo and TMG3tren is tris(2-(N-tetramethylguanidyl)ethyl)amine, and proposed the involvement of [(TMG3tren)NiIII = O] species in alkane hydroxylation reaction. Further, the formation of [NiIV (O)(TMG3tren)](OTf) in the reaction of [Ni(TMG3tren)(OTf)](OTf) with m-CPBA has been detected by using ESI-MS [86]. The time course of the TON for 2 in cyclohexane oxidation is shown in Fig. 3 and Table 4, which clearly indicates that the catalytic activity of the Ni(II) complexes gradually proceeds even after 30 min. Also, the catalytic activity increases from the total TON of 194–673 with the alcohol selectivity (A/K) increasing from 4.5 to 8.0. The good A/K value observed indicates the absence of any free radical auto-oxidation. However, when the number of equivalents of m-CPBA is increased with an aim to increase the concentration of the intermediate and hence the reactivity, the amount of e-caprolactone formed increases as a result of secondary oxidation of cyclohexanone by the increased oxidant concentration. It is interesting to compare the catalytic activity of 1 towards hydroxylation of cyclohexane with those of related mixed ligand nickel(II) complexes 2–5 under identical conditions.

105


3.4.2. Effect of ligand donors Upon replacing the L1 ligand in 1 with phen to give 4, the oxidation of cyclohexane proceeds with increase in catalytic activity from 557 TON to 585 (cyclohexanol, 489 TON; cyclohexanone, 20 TON; e-caprolactone, 76 TON), but with decrease in alcohol selectivity (A/K, 5.1). The replacement of L1 with two sterically hindered –NMe2 donors in 1 by phen with two strongly p-back bonding heterocyclic nitrogen donors to give 4 tends to stabilize the reactive intermediate species and hence the small increase in catalytic activity. This is in agreement with the earlier observation of high catalytic activity for [Ni(TPA)(OAc)(H2O)](BPh4) with three strongly p-back bonding pyridyl nitrogen donors [69,73,74]. We have also shown that the replacement of one of the pyridyl nitrogen donors in this complex by the sterically hindered –NMe2 nitrogen or weakly p-back bonding imidazolyl nitrogen decreases the catalytic activity, illustrating the importance of p-back bonding by heterocyclic nitrogen donor in stabilizing the reactive intermediate [(L)(PA)NiII-O] species [73]. Upon replacing phen in 4 with bpy to give 3 the oxidation of cyclohexane occurs with large decrease in catalytic activity (244 TON; cyclohexanol, 208 TON; cyclohexanone, 12 TON; e-caprolactone, 24 TON), the alcohol selectivity remaining almost constant (A/K, 5.8). As the planarity of bpy (3) is less than that of phen (4), less effective p-back bonding occurs on moving from 4 (b, 5.7) to 3 (b, 3. 2), leading to destabilization of the reactive intermediate species and hence the decrease in catalytic activity. Also, it is well-known that phen complexes are more stable than bpy complexes, which will also enhance the stability of the complex species in solution. Similarly, upon replacing phen in 4 with 2,9-dmp to give 5, the oxidation of cyclohexane takes place with a significant decrease in the catalytic activity from 557 TON to 262 (cyclohexanol, 250 TON; cyclohexanone, 13 TON; e-caprolactone, 24 TON), the alcohol selectivity remaining almost the same (A/K, 6.8). Upon incorporating two methyl groups on 2,9 positions of phen in 4 to obtain 5, the presence of sterically hindering methyl groups renders both r- and p-back bonding less effective leading to a decrease in stability of the proposed reactive intermediate (cf. above). Also, the presence of only one replaceable acetonitrile in 5 (cf. above), in contrast to two in 3 and 4 would also decrease the ligand exchange process with the oxidant m-CPBA. Thus both the Lewis acidity of the Ni(II) center, as modified by the ligand nitrogen donor, and ligand p-back

Cyclohexane

Adamantane

Cumene

700 600 500

Total TON

3.4.1. Effect of ligand denticity Upon replacing the bidentate ligand L1 in 1 by the tridentate ligand L2 to give 2, the oxidation of cyclohexane occurs with a small increase in the total TON, from 557 to 569 (cyclohexanol, 498 TON; cyclohexanone, 17 TON; e-caprolactone, 54 TON) and the alcohol selectivity (A/K, 7.0) remaining the same, which is in accordance with the small change in the covalency parameter (b: 1, 3.5; 2, 2.3). This is expected of the lower Lewis acidity of the Ni(II) center in 2 conferred by the increase in ligand denticity from two in 1 to three in 2. A lower Lewis acidity enhances the ligand exchange process with the oxidant m-CPBA, and also facilitates the O–O bond cleavage in the intermediate species [70]. A similar enhancement in TON has been observed earlier; the increase in denticity of the 3N ligand N-benzyl-bis(2-pyridylmethyl)amine (BzPym2) in [Ni(BzPym2)(OAc)2(H2O)](BPh4) (524 TON) from three to four of the 4N ligand TPA in [Ni(TPA)(OAc)(H2O)](BPh4) (656 TON) leads to enhancement in catalytic activity is increased [70]. So it is clear that increase in denticity encourages not only binding of the oxidant but also enhances the stability and hence the concentration of the intermediate species formed by the mixed ligand complex in solution. It is also possible that the coordinated terminal – NMe2 group of L2 accepts the proton of m-CPBA and enhances the reactivity.

400 300 200 100 0

0

1

2

3

4

5

6

Complex Fig. 4. The total TON of Cyclohexane, Adamantane, Cumene catalyzed by the Ni(II) complexes.

bonding stabilize the reactive intermediate species tune the catalytic efficiency of [Ni(3NO)X2]+ complexes.

3.4.3. Adamantane oxidation The catalytic activity of the Ni(II) complexes towards oxidation of adamantane has been also explored and the results are summarized in Table 6. All the complexes catalyze the oxidation of adamantane efficiently to give 1-adamantanol and 2-adamantanol as the major products along with 2-adamantanone as the minor product. Complex 1 catalyzes the adamantane oxidation to give total TON 548 (1-adamantanol, 425 TON; 2-adamantanol, 123 TON) with a good bond selectivity (3°/2°, 10.4). Interestingly, 2 catalyzes the oxidation of adamantane with the total TON (637: 1-adamantanol, 482 TON; 2-adamantanol, 113 TON; 2-adamantanone, 42 TON) higher than that for 1 but with a slight decrease in bond selectivity (3°/2°, 9.3). Upon increasing the denticity from two in 1 to three in 2 the Ni(II) complex increases in stability, leading to increase in concentration of the reactive intermediate species and hence the catalytic activity, as observed for the catalytic oxidation of cyclohexane (cf. above). A higher catalytic activity is observed also for 4 with the total TON 612 (1-adamantanol, 471 TON; 2-adamantanol, 141 TON), the selectivity remaining almost the same (3°/2°, 10.0), as for cyclohexane oxidation. This provides support to the importance of ligand p-back bonding in causing enhanced stabilization of the reactive intermediate (cf. above). Interestingly, 2-adamantanone product is not observed revealing that 4 catalyzes the oxidation of adamantane very selectively. Thus, when the strongly p-back bonding phen (4) is replaced by the non-planar bpy (3) the total TON decreases to 502 (1-adamantanol, 381 TON; 2-adamantanol, 98 TON; 2-adamantanone, 23 TON) with a slight decrease in bond selectivity (3°/2°, 9.4), as observed for cyclohexane oxidation (Fig. 4), again lending support to the vital role played by p-back bonding (as in 4) in stabilizing the reactive intermediate. Similarly, 5 catalyzes the oxidation of adamantane with a decreased total TON 339 (1-adamantanol, 280 TON; 2-adamantanol, 37 TON; 2-adamantanone, 22 TON) but interestingly with a better bond selectivity (3°/2°, 14.2). Upon introduction of two methyl groups on 2,9-positions of phen ring the catalytic activity decreases up to 45% revealing that the sterically hindering methyl groups effectively lead to the enhanced destabilization of the reactive intermediate and hence the significant decrease in catalytic activity. All the complexes catalyze the oxidation with good TON and high 3°/2° ratio. The high 3°/2° bond selectivity observed indicates that the [(PA)(L)(CH3CN)Ni-O]+ species is the metal-based oxidant in adamantane oxidation also. Thus, interestingly, p-back bonding is one of the predominant factors dictating the stability and selectivity of the reactive intermediate species.

106


The catalytic activity of the Ni(II) complexes for the oxidation of cumene was also investigated and the results are summarized in Table 7. All the Ni(II) complexes catalyse the oxidation of cumene selectively to form 2-phenyl-2-propanol without any side product formation and the trend in catalytic activity is the same as that observed for cyclohexane and adamantane oxidation. All the above observations reveal that ligand p-back bonding, Lewis acidity of nickel(II) center and ligand denticity are the factors that influence the catalytic activity. Systematic studies on well-defined nickel(II) complexes with varying ligand donor atoms are essential to throw more light on the influence of various ligand factors.

[6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20]

4. Conclusions

[21] [22] [23] [24] [25] [26]

A new family of mixed ligand nickel(II) complexes of the type [Ni(PA)(L)(CH3CN)n]+, where H(PA) is picolinic acid and L is bidentate 2N or tridentate 3N co-ligand, has been isolated and characterized as to possess a distorted octahedral coordination geometry. All the complexes catalyze the hydroxylation of alkanes like cyclohexane, adamantane and cumene using m-CPBA as oxidant. An enhancement in ligand denticity by replacing the 2N ligand by a 3N ligand enhances the catalytic activity, illustrating the significance of increased ligand denticity and decreased Lewis acidity of the Ni(II) center in determining the catalytic activity. Also, upon replacing the sterically hindering –NMe2 groups of bidentate 2N co-ligand, by the strongly p-back bonding phen co-ligand, there is a small increase in catalytic activity. However, upon replacing phen by non-planar bpy or 2,9-dmp with sterically hindering methyl groups on 2,9-positions the catalytic activity decreases, supporting the importance of ligand p-back bonding in stabilizing the intermediate [(L)(PA)Ni-O] oxidant species. The observed variation in alcohol selectivity for cyclohexane and bond selectivity for adamantane oxidation illustrates the significance of Lewis acidity of the Ni(II) center, ligand denticity and p-back bonding. Acknowledgments We sincerely thank the Council of Scientific and Industrial Research, New Delhi, for a Senior Research Fellowship to M.S. This work was also supported by Indo-French Centre (IFCPAR) [Scheme No. IFC/A/4109-1/2009/993]. We also thank the Director, National Institute for Interdisciplinary Science and Technology (NIIST)-CSIR, Trivandrum, for providing the computational facility and Dr. C. H. Suresh, CSIR-NIIST, Trivandrum, for fruitful discussions. Professor M. Palaniandavar is a recipient of DST Ramanna Fellowship [Scheme No. SR/S1/RFIC-01/2007 and SR/S1/RFIC-01/2010]. We thank Dr. Balachandran Unni Nair, Central Leather Research Institute, Chennai for providing the ESI-MS facility. Appendix A. Supplementary material CCDC 917086 contains the supplementary crystallographic data for 2. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.ica.2013.07.031. References [1] D. Riley, M. Stern, J. Ebner, in: D.H.R. Barton, A.E. Martell, D.T. Sawyer (Eds.), The Activation of Dioxygen and Homogeneous Catalytic Oxidation, Plenum, New York, 1993, p. 31. [2] M. Costas, K.K. Chen, L. Que Jr., Coord. Chem. Rev. 200 (2000) 517. [3] D.H.R. Barton, D. Doller, Acc. Chem. Res. 25 (1992) 504. [4] A.E. Shilov, A.A. Shteinman, Acc. Chem. Res. 32 (1999) 763. [5] P. Stavropoulos, R. Çelenligil-Çetin, A.E. Tapper, Acc. Chem. Res. 34 (2001) 745.

[27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38]

[39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62]

[63] [64] [65] [66] [67] [68]

T. Punniyamurthy, S. Velusamy, J. Iqbal, Chem. Rev. 105 (2005) 2329. J. Colby, H. Dalton, R. Whittenbury, Annu. Rev. Microbiol. 33 (1979) 481. H. Dalton, Adv. Appl. Microbiol. 26 (1980) 71. J.B. Vincent, G.L. Olivier-Lilley, B.A. Averill, Chem. Rev. 90 (1990) 1447. J.D. Lipscomb, Annu. Rev. Microbiol. 48 (1994) 317. A.L. Feig, S.J. Lippard, Chem. Rev. 94 (1994) 759. B.J. Waller, J.D. Lipscomb, Chem. Rev. 96 (1996) 2625. M. Baik, M. Newcomb, R.A. Friesner, S.J. Lippard, Chem. Rev. 103 (2003) 2385. M. Costas, M.P. Mehn, M.P. Jensen, L. Que Jr., Chem. Rev. 104 (2004) 939. E.Y. Tshuva, S.J. Lippard, Chem. Rev. 104 (2004) 987. S.V. Kryatov, E.V. Rybak-Akimova, S. Schindler, Chem. Rev. 105 (2005) 2175. J.H. Green, H. Dalton, J. Biol. Chem. 264 (1989) 17698. N. Kitajima, H. Fukui, Y. Moro-oka, J. Chem. Soc., Chem. Commun. (1988) 485. W. Nam, R. Ho, J.S. Valentine, J. Am. Chem. Soc. 113 (1991) 7052. N. Kitajima, M. Ito, H. Fukui, Y. Moro-oka, J. Chem. Soc., Chem. Commun. (1991) 102. M. Kodera, H. Shimakoshi, K. Kano, Chem. Commun. (1996) 1737. K. Chen, L. Que Jr., Chem. Commun. (1999) 1375. K. Chen, L. Que Jr., Angew. Chem., Int. Ed. 38 (1999) 2227. Y. Jin, J.D. Lipscomb, J. Biol. Inorg. Chem. 6 (2001) 717. K. Chen, L. Que Jr., J. Am. Chem. Soc. 123 (2001) 6327. Y. Mekmouche, S. Ménage, C. Toia-Duboc, M. Fontecave, J.-B. Galey, C. Lebrun, J. Pécaut, Angew. Chem., Int. Ed. 40 (2001) 949. W. Nam, J.Y. Ryu, I. Kim, C. Kim, Tetrahedron Lett. 43 (2002) 5487. K. Chen, M. Costas, L. Que Jr., J. Chem. Soc., Dalton Trans. (2002) 672. W.J. Song, Y.O. Ryu, R. Song, W. Nam, J. Biol. Inorg. Chem. 10 (2005) 294. G. Xue, R.D. Hont, E. Münck, L. Que Jr., Nat. Chem. 2 (2010) 400. H. Komatsuzaki, N. Sakamoto, M. Satoh, S. Hikichi, M. Akita, Y. Moro-oka, Inorg. Chem. 37 (1998) 6554. M.S. Seo, J.Y. Kim, J. Annaraj, Y. Kim, Y. Lee, S. Kim, J. Kim, W. Nam, Angew. Chem., Int. Ed. 46 (2007) 377. K. Nehru, S.J. Kim, I.Y. Kim, M.S. Seo, Y. Kim, S. Kim, J. Kim, W. Nam, Chem. Commun. (2007) 4623. F.A. Chavez, P.K. Mascharak, Acc. Chem. Res. 33 (2000) 539. W. Nam, I. Kim, Y. Kim, C. Kim, Chem. Commun. (2001) 1262. N. Kitajima, K. Fujisawa, Y. Moro-oka, J. Am. Chem. Soc. 111 (1989) 8975. R.R. Jacobson, Z. Tyeklar, A. Farooq, K.D. Karlin, S. Liu, J. Zubieta, J. Am. Chem. Soc. 110 (1988) 3690. N.W. Aboelella, S.V. Kryatov, B.F. Gherman, W.W. Brennessel, V.G. Young Jr., R. Sarangi, E.V. Rybak-Akimova, K.O. Hodgson, B. Hedman, E.I. Solomon, C.J. Cramer, W.B. Tolman, J. Am. Chem. Soc. 126 (2004) 16896. L.M. Mirica, X. Ottenwaelder, T.D.P. Stack, Chem. Rev. 104 (2004) 1013. L. Que Jr., W.B. Tolman, Angew. Chem., Int. Ed. 41 (2002) 1114. M. Suzuki, Acc. Chem. Res. 40 (2007) 609. C.J. Cramer, W.B. Tolman, Acc. Chem. Res. 40 (2007) 601. S. Itoh, S. Fukusumi, Acc. Chem. Res. 40 (2007) 592. T. Kojima, Chem. Lett. (1996) 121. T. Kojima, H. Matsuo, Y. Matsuda, Inorg. Chim. Acta 300–302 (2000) 661. J.R. Bryant, T. Matsuo, J.M. Mayer, Inorg. Chem. 43 (2004) 1587. M. Yamaguchi, H. Kousaka, S. Izawa, Y. Ichii, T. Kumano, D. Masui, T. Yamagishi, Inorg. Chem. 45 (2006) 8342. T. Kojima, K. Hayashi, S. Iizuka, F. Tani, Y. Naruta, M. Kawano, Y. Ohashi, Y. Hirai, K. Ohkubo, Y. Matsuda, S. Fukuzumi, Chem. Eur. J. 13 (2007) 8212. T. Okumura, Y. Morishima, H. Shiozaki, T. Yagyu, Y. Funahashi, T. Ozawa, K. Jitsukawa, H. Masuda, Bull. Chem. Soc. Jpn. 80 (2007) 507. S. Yiu, W. Man, T. Lau, J. Am. Chem. Soc. 130 (32) (2008) 10821. D. Schröder, H. Schwarz, Angew. Chem., Int. Ed. Engl. 34 (1995) 1973. K. Jitsukawa, Y. Oka, S. Yamaguchi, H. Masuda, Inorg. Chem. 43 (2004) 8119. M. Murali, R. Mayilmurugan, M. Palaniandavar, Eur. J. Inorg. Chem. 22 (2009) 3238. J.F. Kinneary, J.S. Albert, C.J. Burrows, J. Am. Chem. Soc. 110 (1988) 6124. S. Hikichi, M. Yoshizawa, Y. Sasakura, M. Akita, Y. Moro-oka, J. Am. Chem. Soc. 120 (1998) 10567. K. Shiren, S. Ogo, S. Fujinami, H. Hayashi, M. Suzuki, A. Uehara, Y. Watanabe, Y. Moro-oka, J. Am. Chem. Soc. 122 (2000) 254. B.S. Mandimutsira, J.L. Yamarik, T.C. Brunold, W. Gu, S.P. Cramer, C.G. Riordan, J. Am. Chem. Soc. 123 (2001) 9194. S. Hikichi, M. Yoshizawa, Y. Sasakura, H. Komatsuzaki, Y. Moro-oka, M. Akita, Chem. Eur. J. 7 (2001) 5011. K. Fujita, R. Schenker, W. Gu, T.C. Brunold, S.P. Cramer, C.G. Riordan, Inorg. Chem. 43 (2004) 3324. J. Cho, H. Furutachi, S. Fujinami, M. Suzuki, Angew. Chem., Int. Ed. 43 (2004) 3300. J. Cho, H. Furutachi, S. Fujinami, M. Suzuki, Angew. Chem. 116 (2004) 3362. M.T. Kieber-Emmons, J. Annaraj, M.S. Seo, K.M. Van Heuvelen, T. Tosha, T. Kitagawa, T.C. Brunold, W. Nam, C.G. Riordan, J. Am. Chem. Soc. 128 (2006) 14230. M.T. Kieber-Emmons, C.G. Riordan, Acc. Chem. Res. 40 (2007) 618. S. Yao, E. Bill, C. Milsmann, K. Wieghardt, M. Driess, Angew. Chem., Int. Ed. 47 (2008) 7110. S. Yao, E. Bill, C. Milsmann, K. Wieghardt, M. Driess, Angew. Chem. 120 (2008) 7218. A. Company, S. Yao, K. Ray, M. Driess, Chem. Eur. J. 16 (2010) 9669. S. Hikichi, C. Kobayashi, M. Yoshizawa, M. Akita, Chem. Asian J. 5 (2010) 2086. T. Tano, Y. Doi, M. Inosako, A. Kunishita, M. Kubo, H. Ishimaru, T. Ogur, H. Sugimoto, S. Itoh, Bull. Chem. Soc. Jpn. 83 (2010) 530.

M. Sankaralingam et al. / Inorganica Chimica Acta 407 (2013) 98–107 [69] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81]

T. Nagataki, Y. Tachi, S. Itoh, Chem. Commun. (2006) 4016. T. Nagataki, K. Ishii, Y. Tachi, S. Itoh, Dalton Trans. (2007) 1120. T. Nagataki, S. Itoh, Chem. Lett. 36 (6) (2007) 748. S. Hikichi, H. Okuda, Y. Ohzu, M. Akita, Angew. Chem., Int. Ed. 48 (2009) 188. M. Balamurugan, R. Mayilmurugan, E. Suresh, M. Palaniandavar, Dalton Trans. 40 (2011) 9413. M. Sankaralingam, M. Balamurugan, M Palaniandavar, (Submitted for publication). G.M. Sheldrick, SAINT 5.1 ed., Siemens Industrial Automation Inc., Madison, WI, 1995. SADABSEmpirical Absorption Correction Program, University of Göttingen, Göttingen, Germany, 1997. G.M. Sheldrick, SHELXTL Reference Manual: Version 5.1, Bruker AXS, Madison, WI, 1997. G.M. Sheldrick, SHELXL-97: Program for Crystal Structure Refinement, University of Göttingen, Göttingen, Germany, 1997. M. Ganeshpandian, R. Loganathan, S. Ramakrishnan, A. Riyasdeen, M.A. Akbarsha, M. Palaniandavar, Polyhedron 52 (2013) 929. K.L. Klotz, L.M. Slominski, A.W. Hull, V.M. Gottsacker, R.M. Ballesté, L. Que Jr., J.A. Halfen, Chem. Commun. (2007) 2063. M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G.A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H.P. Hratchian, A.F. Izmaylov, J. Bloino, G. Zheng, J.L. Sonnenberg, M.

[82] [83]

[84]

[85] [86]

107

Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J.A. Montgomery Jr., J.E. Peralta, F. Ogliaro, M. Bearpark, J.J. Heyd, E. Brothers, K.N. Kudin, V.N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J.C. Burant, S.S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J.M. Millam, M. Klene, J.E. Knox, J.B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, R.L. Martin, K. Morokuma, V.G. Zakrzewski, G.A. Voth, P. Salvador, J.J. Dannenberg, S. Dapprich, A.D. Daniels, Ö. Farkas, J.B. Foresman, J.V. Ortiz, J. Cioslowski, D.J. Fox, GAUSSIAN 09, Revision A.1, Gaussian, Inc., Wallingford CT, 2009. M. Velusamy, M. Palaniandavar, K.R. Justin Thomas, Polyhedron 17 (1998) 2179. (a) A.B.P. Lever, Inorganic Electronic Spectroscopy, Second ed., Elsevier, Amsterdam, 1986; (b) A.B.P. Lever, J. Chem. Educ. 45 (1968) 711. (a) M. Ciampolini, Inorg. Chem. 5 (1966) 35; (b) A.T. Phillip, W. Mazurek, A.T. Casey, Aust. J. Chem. 24 (1971) 501; (c) S.V. Kryatov, B.S. Mohanraj, V.V. Tarasov, O.P. Kryatova, E.V. RybakAkimova, B. Nuthakki, J.F. Rusling, R.J. Staples, Inorg. Chem. 41 (2002) 923. A. Bravo, H.-R. Bjorsvik, F. Fontana, F. Minisci, A. Serri, J. Org. Chem. 61 (1996) 9409. F.F. Pfaff, F. Heims, S. Kundu, S. Mebs, K. Ray, Chem. Commun. 48 (2012) 3730.